The production of bleached chemical pulp is a major industry around the world. More than 50 million tons of bleached pulp is produced annually. Bleached chemical pulp is the largest component of all types of white paper, including that used in photocopy paper, writing paper, and paper packaging. In addition, bleached chemical pulp is also used to impart strength to less expensive grades of paper, such as newsprint. Bleached chemical pulp has large markets because of its high degree of whiteness and cleanliness, the stability of the whiteness, its high strength, and the ease and uniformity of the printing surface it provides. These attributes are obtained when lignin, which is colored and decreases the interfiber bonding of the cellulose, is almost completely removed from the pulp.
In the process of chemical pulping, the furnish (or feedstock) primarily consists of wood chips which are added to a reaction chamber, known as a digester, and are treated with chemicals to dissolve lignin in the pulp. There are several chemical pulping processes known in the art. Two of the major chemical pulping processes are kraft pulping, in which the pulp is cooked in alkaline liquor, and sulfite pulping, in which the pulp is cooked in acidic liquor. Both kraft pulping and sulfite pulping may be performed in batch or continuous digestors.
One of the main purposes of the pulping process is to release lignin which binds cellulose fibers in the feedstock. Pulping dissolves 85% to 95% of the lignin in the feedstock material. Following the pulping stage, the pulp is washed with water to remove dissolved lignin.
While pulping removes most of the lignin in the feedstock material, it is not capable of removing all the lignin without destroying the cellulose fibers of the feedstock. The remaining lignin is removed from the pulp by bleaching.
A pulp bleaching process may consist of many stages. For example, following pulping, a pulp bleaching process may comprise an alkaline oxygen delignification stage (O), an enzymatic treatment stage (X), one or more chlorine dioxide bleaching stages (D), and one or more alkaline extraction stages (E). A pulp bleaching process may also comprise one or more water washes or alternatively, each stage may comprise a water wash as a final step of the stage. Thus, a representative pulp bleaching sequence in which pulp is bleached using three chemical bleaching stages and two alkaline extraction stages may be represented as D-E-D-E-D. Similarly, a pulp bleaching sequence wherein pulp is subjected to an alkaline oxygen delignification stage, an enzymatic treatment stage, three chlorine dioxide bleaching stages and two alkaline extraction stages wherein each stage is followed by a water wash may be represented by O-X-D-E-D-E-D.
It is common for mills to perform an alkali-oxygen delignification stage prior to carrying out chemical bleaching of pulp. This process consists of reacting the pulp with oxygen and alkali at high temperatures (approximately 100° C.) for a period of about one hour. Alkali-oxygen delignification reduces the amount of lignin in the pulp by 35-50%, but this process is harsh on the pulp and is often accompanied by destruction of some of the cellulose fibers in the pulp. Following alkali-oxygen delignification, the pulp is washed as described earlier to remove solubilized lignin.
The next bleaching stage after alkali-oxygen delignification is usually chemical bleaching with oxidative chemicals, the most prominent being chlorine dioxide (ClO2). However, several processes have been described which may facilitate or enhance bleaching of pulp prior to chemical bleaching. For example, an enzymatic treatment stage with xylanase may be used to enhance the bleaching of pulp prior to chemical bleaching.
Xylanases are used in the pulp and paper industry to enhance the bleaching of pulp and to decrease the amount of chlorinated chemicals used in bleaching stages (Eriksson, 1990; Paice et al., 1988; Pommier et al., 1989). There have been several mechanisms proposed for the bleaching action of xylanase. One is that lignin is connected to crystalline cellulose through xylan and xylanase enzymes facilitate bleaching of pulp by hydrolysing xylan, releasing coloured lignin from the pulp. A second proposed mechanism is that xylanase removes xylan thereby improving the alkali extractability of the lignin. Regardless of the mechanism, xylanase treatment allows subsequent bleaching chemicals such as chlorine, chlorine dioxide, hydrogen peroxide, or combinations of these chemicals to bleach pulp more efficiently than in the absence of xylanase. Pretreatment of pulp with xylanase prior to chemical bleaching increases the whiteness and quality of the final paper product and reduces the amount of chlorine-based chemicals which must be used to bleach the pulp. This in turn decreases the chlorinated effluent produced by such processes.
Xylanases have been isolated from a variety of organisms including bacteria and fungi. Generally, fungal xylanases exhibit optimal activity at acidic pHs, in the range of about 3.5 to 5.5, and a temperature of about 50° C. In contrast, bacterial xylanases exhibit optimal activity at pH 5 to pH 7 and a temperature optimum between 50° C. and 70° C.
Following kraft pulping and alkali oxygen delignification the temperature and the pH of the pulp are high, and each of these operations must be followed by a water wash. The conditions of the pulp following pulping and alkali oxygen delignification have prompted efforts to identify and isolate thermophilic and alkalophilic xylanases which may be used for enzymatic treatment with minimal adjustment of the temperature and pH of the pulp. For example, U.S. Pat. No. 5,405,789 to Campbell et al., discloses construction of thermostable mutants of low molecular mass xylanase from Bacillus circulans. U.S. Pat. No. 5,759,840 to Sung et al., discloses modification of a family 11 xylanase from Trichoderma reesei to improve thermophilicity, alkalophilicity and thermostability as compared to the natural xylanase. U.S. Pat. No. 5,916,795 to Fukunaga et al., discloses a thermostable xylanase from Bacillus. A publication entitled “Xylanase Treatment of Oxygen-Bleached Hardwood Kraft Pulp at High Temperature and Alkaline pH Levels Gives substantial Savings in Bleaching Chemicals” to Shah et al., (J. of Pulp and Paper Science, vol 26 No. 1 Jan. 2000, which is herein incorporated by reference) discloses treating oxygen delignified hardwood pulp with xylanase from Thermotoga maritima at pH 10 and 90° C. and subsequently bleaching the pulp. These documents disclose alkalophilic or thermophilic xylanases, and suggest the use of xylanases to enzymatically treat pulp prior to the first chlorine dioxide bleaching stage. None of these documents suggest using xylanases after the first chlorine dioxide bleaching stage.
The next stage in a typical pulp bleaching process is usually chlorine dioxide bleaching with chlorine dioxide, chlorine or in some instances, a combination of chlorine dioxide and other oxidative bleaching agents. For example, the first chlorine dioxide stage in a chemical bleaching process is often called the Do or D100 stage. Subsequent chlorine dioxide bleaching stages are referred to as D1, D2 and so on. For mills that bleach pulp without an alkali-oxygen delignification stage, the Do stage is the first chemical bleaching stage. The Do stage is usually carried out at pH 1.5 to 3.0. In a small but decreasing number of mills, up to 30% to 50% chlorine gas may be added to ClO2 in an effort to achieve a higher efficiency of lignin removal. Such a stage is referred to as a CD stage. After a Do or CD stage, the pulp is washed with water, and alkaline extracted. Alkaline extraction is carried out by adjusting the pH of the pulp to 9.0 to 12.0 with sodium hydroxide or sodium carbonate at a temperature between 60° C. to 120° C. and maintaining the pulp at these conditions for a period of 30 to 90 minutes. The pH may drift by 0.5 to 2.0 pH units depending on the initial pH and the pH of the pulp and is usually not adjusted during the alkaline extraction stage. After the alkaline extraction stage, the pulp is washed with water. The chlorine dioxide bleaching stage, wash and alkaline extraction is repeated until the pulp is suitably bleached. In most cases, two to three rounds of bleaching, alternating between chlorine dioxide stages and alkaline extraction stages, is required before the pulp is suitably bleached.
In all commercial applications, xylanase use within a pulp bleaching sequence comprises a xylanase treatment stage followed by one or more chemical bleaching stages. This usually results in a pulp with increased brightness compared to pulp treated in a similar manner but without xylanase treatment. Alternatively, a specific brightness level can be achieved using a smaller amount of bleaching chemicals when the pulp is treated with xylananse prior to bleaching, compared to pulp that is not treated with xylanase before bleaching.
Unfortunately, there are difficulties associated with xylanase treatment prior to the first chlorine dioxide bleaching stage. The application of xylanase to pulp requires proper mixing of enzyme with pulp, pH control, temperature control, enzyme dosage control, and residence time control. Mill equipment which is used prior to the first chlorine dioxide bleaching stage usually consists of a brownstock decker, stock pump and storage tower. This equipment is not designed to control such complex parameters. For example, most stock pumps are incapable of adequately mixing enzyme and pulp. Also, the storage tower described above is not constructed to hold pulp for a fixed time period and pulp often channels through the tower. Further, as xylanase treatment must be carried out at moderate pH levels, acid is required to reduce the pH of the pulp following kraft pulping. This equipment is usually not built to withstand the addition of acids and thus, corrosion of mill equipment is an important concern. In addition, the storage and use of acids can create a potentially hazardous environment for mill workers, and such an environment may require implementing specialized safety precautions which could increase the cost of pulp bleaching above and beyond the cost of acid. Other problems with enzyme treatment include the lack of instrumentation and inability to sample pulp in brownstock storage towers, which makes process control difficult. The addition of chemicals in the bleach plant depends on the kappa number of the pulp, the brightness of the pulp, and the final pulp brightness desired, all of which are affected by enzyme treatment.
U.S. Pat. No. 5,645,686 discloses a process for bleaching a chemical paper pulp by means of a sequence of treatment stages involving at least one stage with hydrogen peroxide and at least one stage with a peroxyacid. The patent also discloses a xylanase treatment stage in combination with the pulp bleaching sequence. The patent does not suggest treating pulp with xylanase treatment stage after a chlorine dioxide stage in a pulp bleaching process that employs only chlorine dioxide bleaching stages. Further, there is no teaching as to whether a xylanase treatment stage after a first chlorine dioxide bleaching stage may be more effective in enhancing the bleaching of pulp compared to a pulp bleaching sequence wherein xylanase treatment is performed prior to the first chlorine dioxide bleaching stage.
WO 91/05908 discloses a process for producing bleached lignocellulosic pulp having reduced organically bound chlorine and reduced brightness reversion. The process entails treating pulp with xylanase after a chlorination stage which primarily employs chlorine. Wong et al., (2000. J. of Pulp and Paper Science Vol 26 No. 10 377-383, which is herein incorporated by reference) teaches a xylanase treatment stage following complete chemical bleaching.
The drawbacks associated with implementing a xylanase treatment stage after the first chlorine dioxide bleaching stage are similar to the drawbacks associated with implementing a xylanase treatment stage prior to the first chlorine dioxide bleaching stage, including the costs and safety concerns of using acids, and the difficulty maintaining, monitoring and controlling the process. Incorporating a separate xylanase treatment stage after chlorine dioxide bleaching requires purchasing a suitable vessel to carry out the treatment. Most mills do not have the money or space to add an additional vessel and thus, incorporating a separate xylanase treatment stage after a chlorine dioxide bleaching stage may not be economical or feasible. Presently, there are no known mills which carry out an enzyme treatment stage after chlorine dioxide bleaching.
While the bleaching processes known in the art generally result in adequate pulp bleaching, there is a need in the art to increase the efficiency and safety of bleaching. Further, the pulp industry is under pressure to decrease the use of chlorine-containing bleaching chemicals, such as chlorine and chlorine dioxide, and thus, any method or process which can be integrated into a pulp bleaching process to reduce the use of chlorine-containing bleaching chemicals or the toxic effluents produced by the use of such chemicals would be an important and valuable asset to the pulp industry.
There is a need in the prior art for novel methods and more efficient methods of bleaching pulp. Further there is a need in the art for methods, or processes which can be integrated into existing pulp bleaching processes to increase the efficiency of bleaching and reduce the use of chlorine containing bleaching compounds or the toxic effluents produced by the use of such chemicals.
It is an object of the invention to overcome drawbacks in the prior art.
The above object is met by a combination of the features of the main claims. The sub claims disclose further advantageous embodiments of the invention.